1. Field of the Invention
This invention is in the field of strong acids useful for catalysis.
2. Description of Related Art
Trifluoromethanesulfonic acid is used for catalysis where a strong acid is needed. It offers a safer, more easily handled alternative to the inorganic acids, hydrogen fluoride and sulfuric acid, which are widely used in industrial processes. Known hydrofluoroalkanesulfonic acids, such as tetrafluoroethanesulfonic acid (TFESA), could be more effective candidates to replace trifluoromethanesulfonic acid in catalytic applications.
Hydrofluoroalkanesulfonic acids are made by the addition of the elements of sulfurous acid, H2SO3, to fluoroolefins. For example, TFESA is the product of the reaction with tetrafluoroethylene (TFE):CF2═CF2+H2SO3→HCF2—CF2SO3H  (1)In practice, the fluoroolefin is reacted with aqueous sulfite solution, usually an alkali metal sulfite. The solution is buffered to suppress the competing reaction, hydration of the fluoroolefin to form a carboxylic acid byproduct. In the case of TFE, the acid resulting from hydration is difluoroacetic acid, HCF2CO2H.
The literature does not provide an efficient manufacturing method for this reaction. An early reference, U.S. Pat. No. 2,403,207 (1946), teaches the optional use of free radical initiator. A. Kilian and H. Waeschke in Wissenschäftliche Beiträge, Ingenieurhochschule Köthen, pp. 22-28 (1978), teach the utility of peroxide initiator for the reaction. Borax is generally used as the buffer. Extraction, usually with ethanol, is used to recover the product salt or acid. Typically, the reaction mixture is worked up by drying, followed by extraction with hot ethanol. The extract is dried to remove the ethanol and the resulting solid is treated with sulfuric acid. This mixture is distilled to yield the hydrofluoroalkanesulfonic acid, usually as a hydrate if any water is present, and byproduct organic acid or acetate. If the unhydrated hydrofluoroalkanesulfonic acid is needed, a further step, such as treatment with thionyl chloride, is necessary. As late as 1995, Chinese Patent Application 1097191 noted the shortcomings of the available manufacturing methods (“harsh reaction conditions, low yield, low reaction uniformity and high cost”) and proposed replacement of water with aqueous organic solutions and the use of organic ammonium compounds in the reaction. These proposed modifications make the reaction more complex and expensive. Two years later and fifty years after the '207 patent, Japanese Patent Application 9-104686 (1997) disclosed preparative examples of the reactions with TFE and with hexafluoropropylene (HFP) with aqueous sulfite. Reaction times were 110 hours and yields 20% or less.
A simplified process is needed for making hydrofluoroalkanesulfonic acids in good purity and yield.